Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of heredity. His quantitative approach and careful selection of experimental subjects allowed him to deduce laws that govern genetic inheritance.

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Advantages of Pea Plants: Short generation time, large numbers of offspring, and controlled mating (self- or cross-pollination).

Mendel’s Experimental Approach

Mendel tracked characters with two distinct forms and used true-breeding varieties. He performed hybridization by mating contrasting, true-breeding parents (P generation), producing hybrid offspring (F1 generation). Self- or cross-pollination of F1 hybrids produced the F2 generation.

The Law of Segregation

Mendel’s experiments disproved the blending hypothesis. Crossing true-breeding purple and white flowers yielded all purple F1 hybrids. Self-pollination of F1 hybrids produced a 3:1 ratio of purple to white flowers in the F2 generation.

• Dominant trait: Purple flower color.

• Recessive trait: White flower color.

• Gene: Mendel’s “heritable factor” is now known as a gene.

Mendel’s Model of Inheritance

Mendel proposed four concepts to explain the 3:1 ratio:

1. Alleles: Alternative versions of genes account for variations in inherited characters.

2. Inheritance: Each organism inherits two alleles, one from each parent.

3. Dominance: If alleles differ, the dominant allele determines appearance; the recessive allele is masked.

4. Law of Segregation: The two alleles for a character segregate during gamete formation.

Genetic Vocabulary

Understanding genetic terminology is essential for interpreting Mendelian inheritance.

• Homozygote: Organism with two identical alleles for a gene (homozygous).

• Heterozygote: Organism with two different alleles for a gene (heterozygous).

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

The Testcross

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the parent is heterozygous.

The Law of Independent Assortment

Mendel’s second law states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or far apart on the same chromosome.

• Monohybrid cross: Heterozygotes for one character.

• Dihybrid cross: Heterozygotes for two characters.

Probability Laws in Genetics

Mendelian inheritance follows the rules of probability. The multiplication rule calculates the probability of independent events occurring together, while the addition rule calculates the probability of mutually exclusive events.

• Multiplication rule:

• Addition rule:

Complex Patterns of Inheritance

Inheritance patterns can be more complex than simple Mendelian genetics. These include incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

• Incomplete dominance: Heterozygote phenotype is intermediate between two homozygotes.

• Codominance: Both alleles affect the phenotype in separate ways.

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple phenotypic characters.

Epistasis and Polygenic Inheritance

Epistasis occurs when one gene affects the expression of another. Polygenic inheritance involves multiple genes contributing to a single trait, often resulting in quantitative variation.

• Epistasis example: Labrador retriever coat color is determined by two genes.

• Polygenic inheritance example: Human height and skin pigmentation.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype. Phenotypic plasticity describes the ability to produce different phenotypes under varying conditions. Multifactorial traits depend on both genetic and environmental influences.

Mendelian Patterns in Human Genetics

Many human traits follow Mendelian inheritance patterns. Pedigree analysis is used to study inheritance across generations. Recessive and dominant disorders, as well as multifactorial diseases, are important in human genetics.

• Pedigree: Family tree showing inheritance of traits.

• Recessive disorders: Albinism, cystic fibrosis, sickle-cell disease.

• Dominant disorders: Achondroplasia, Huntington’s disease.

• Multifactorial disorders: Heart disease, cancer, alcoholism.

Genetic Testing and Counseling

Genetic counselors provide information to prospective parents. Fetal and newborn testing can reveal genetic disorders, helping families make informed decisions.

Summary Tables

The following tables summarize key relationships among alleles and genes: